Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, generator, gearbox, nacelle, and one or more turbine blades. The turbine blades capture kinetic energy from wind using known foil principles and transmit the kinetic energy through rotational energy to turn a shaft coupling the rotor blades to a gearbox, or if a gearbox is not used, directly to the generator. The generator then converts the mechanical energy to electrical energy that may be deployed to a utility grid.
To ensure that wind power remains a viable energy source, efforts have been made to increase energy outputs by modifying the size and capacity of wind turbines. One such modification has been to increase the length of the turbine blades. In addition, various rotor blades are manufactured with a pre-bend or a tendency to bend at a certain location. Such rotor blades, however, may be more susceptible to striking the tower of the wind turbine. A tower strike can significantly damage a turbine blade and the tower and, in some instances, can even bring down the entire wind turbine.
Devices and systems are known for detecting turbine blade deflection using various types of active or mechanical sensors. For example, U.S. Pat. No. 6,619,918 describes the use of fiber optic strain gauges on the turbine blades to measure load on the blades and to deduce tip clearance as a function of the measured load. Further, U.S. Pat. No. 7,059,822 describes a system wherein beams are coupled to the blades and deflection of the blades is determined as a function of the amount of deflection of the beams. Moreover, U.S. Pat. No. 7,246,991 describes a control system for avoiding a tower strike that uses a signal from a sensor that measures deflection of the turbine blades. Several possible sensor types are described, including strain gauges, accelerometers mounted in the blades, and active radar devices.
The conventional sensors and associated systems are relatively complex and costly, and calibrating such sensors can be quite complex and time consuming. Moreover, the control systems of most pitch systems are only concerned with reliability in ensuring that one out of three blades fails at feather, rather than ensuring that all three blades reliably stay out of an unsafe region (i.e. the region corresponding to a likely tower strike occurrence).
Accordingly, there is a need for an improved system and method for preventing tower strike by a rotor blade of a wind turbine that does not use the aforementioned sensors.